I work with a client who plans three-week sprints. They've recently started taking a divide and conquer approach to planning. It goes something like this:
1. retrospective, all together as one team
2. split up the candidate stories and assign to two or three groups
3. split into groups and plan
But but but! This isn't real scrum!
No, it's not. But it works for them. It's taken a planning process from a painful two days per sprint to a semi-painful 5 hours per sprint. The gain mostly comes from the team being so large - it's about 15 people. That's already about twice the size that many agile practitioners will advocate.
Here's a hint: if you're already violating some of the rules of whatever process you're following, then don't get all stuffy about violating other parts of the rules.
The goal of process is to do what works. If that means you work with a large team, okay. If that means the team trusts each other enough to take their plan, okay.
In other words, be strict or don't. Either way, don't sweat it.